Recently, metallic oxides in composition ranges described by La.sub.2-X A.sub.X CuO.sub.4, where A can be Ba, Sr, or Ca, were found which exhibit superconductivity with transition temperatures T.sub.C as high as 40 K. This sparked a flurry of activity and led to the discovery of another new class of superconducting metallic oxides which can be described by the formula LnBa.sub.2 Cu.sub.3 O.sub.7, where Ln can be Y or any of the trivalent rare earth elements. Several reports of materials with still higher transition temperatures remain to be confirmed.
As a consequence of this work, low cost applications of superconductivity may now be possible. One such application considered is disclosed in a paper by R. H. Koch et al, "Quantum interference devices made from superconducting oxide thin films", Applied Physics Letters, Vol. 51 (3), pp. 200-202 (July 20, 1987).
Koch et al disclose the fabrication of superconducting quantum interference devices (dc SQUID's) from thin films of the superconducting oxide YBa.sub.2 Cu.sub.3 O.sub.Y. The devices were made by first lithographically patterning an ion implant mask containing a 40.times.40 .mu.m loop and two 17-.mu.m-wide weak links over a 1-.mu.m-thick oxide film. An ion beam was used to destroy the superconductivity in the film surrounding the device without actually removing material, resulting in a completely planar structure for the SQUID's.
The authors note that a Josephson tunnel junction device would be preferable to a weak link SQUID, but due to the technical difficulties in making quality junctions and the simplicity in fabricating weak link SQUID's, the latter approach was taken.
It is well-known that Josephson tunnel junction devices offer a much larger variety of applications than weak link SQUID's. Further, most potential applications of high temperature superconductors in advanced microelectronic circuits will require tunnel junctions capable of carrying high critical current densities. Development of reliable techniques for producing such junctions has been a limiting factor even for the use of conventional metal superconductors.
Known techniques have always involved the formation or deposition of a very thin insulating barrier on a superconducting substrate and then depositing another superconducting layer on top of the barrier layer. There appears to be little prospect that these known techniques for producing satisfactory tunnel barriers can be adapted to the new high T.sub.C superconducting oxides, because the latter materials must be annealed at high temperatures following deposition, which will almost certainly destroy any sufficiently thin barrier.